In an image forming apparatus, a semiconductive roller is used as a charging roller for uniformly electrically charging a surface of a photoreceptor body, as a developing roller for developing an electrostatic latent image formed by light-exposing the electrically charged photoreceptor surface into a toner image, as a transfer roller for transferring the formed toner image onto a paper sheet or the like, or as a cleaning roller for removing toner from the photoreceptor surface after the transfer of the toner image.
For improvement of the durability and the compression set properties of the semiconductive roller, the semiconductive roller is preferably formed of a nonporous crosslinking product of a rubber composition.
The semiconductive roller is produced, for example, by extruding the rubber composition into a nonporous tubular body, crosslinking the tubular body, inserting a shaft such as of a metal into a center through-hole of the tubular body, and polishing an outer peripheral surface of the tubular body.
In general, the rubber composition to be used as a material for the semiconductive roller is imparted with ion conductivity by using an ion-conductive rubber (e.g., an epichlorohydrin rubber) as a rubber component to be thereby imparted with semiconductivity as a whole.
Further, a diene rubber is generally used in combination with the ion-conductive rubber as the rubber component.
The diene rubber functions to improve the fluidity and the formability of the rubber composition in the extrusion, and to improve the smoothness of the outer peripheral extrusion surface of the nonporous tubular body produced by the extrusion to make the outer peripheral surface as smooth as possible without irregularity. Further, the diene rubber functions to improve the mechanical strength and the durability of the semiconductive roller and to improve the rubber characteristic properties of the semiconductive roller, i.e., to make the semiconductive roller more flexible and less susceptible to permanent compressive deformation with a reduced compression set. Furthermore, the diene rubber is oxidized by irradiation with ultraviolet radiation or the like, whereby an oxide film serving as a dielectric layer, a lower friction layer or the like is formed in the outer peripheral surface of the semiconductive roller as will be described later.
The diene rubber is excellent in these functions. Advantageously usable as the diene rubber are a butadiene rubber (BR) which is capable of properly electrically charging a positively-chargeable nonmagnetic single-component toner, and a chloroprene rubber (CR) which has the above functions and further functions to improve the flexibility of the semiconductive roller, to increase the nip width to increase the toner charging amount and to reduce the damage to the toner to improve the imaging durability.
If the proportion of the epichlorohydrin rubber is increased, for example, to improve the semiconductivity of the semiconductive roller, or if the proportion of the CR is increased and hence the proportion of the BR is relatively reduced in order to maintain the flexibility and the nip width of the semiconductive roller which tend to be reduced by the increase in the proportion of the epichlorohydrin rubber, however, the outer peripheral extrusion surface is liable to be roughened with the fluidity and the formability of the rubber composition reduced in the extrusion.
Even if the roughened outer peripheral extrusion surface is polished in the subsequent step, the semiconductive roller is problematically liable to cause image density unevenness, for example, when being used as the developing roller for image formation.
Where an automotive tire is produced by extrusion of a rubber composition, for example, a consideration is given to the shape of a back die of an extruder in order to improve an outer peripheral extrusion surface to make the outer peripheral surface as smooth as possible without irregularity (Patent Document 1 and the like).
Where the semiconductive roller or other OA system rubber component is produced by the extrusion, on the other hand, it is a general practice to increase the fluidity and the formability of the rubber composition, for example, by increasing the setting temperature of the extruder or by changing the shape of a die head.